1. Field of the Invention
The present invention relates to a method of stress inducing transformation of austenite stainless steel and methods of producing magnetic members and composite magnetic members.
2. Description of the Related Art
At present, austenite stainless steel is widely used in various fields of railway vehicles to kitchen utensils for domestic use. Therefore, great importance is attached to the mechanical property of austenite stainless steel. Concerning austenite stainless steel, the following are well known. When austenite stainless steel is subjected to cold working in a temperature range from the point Ms to the point Md, the martensite phase is generated from the austenite phase which is a mother phase, so that the stress induced-martensite transformation is caused. In this case, the point Ms is an upper limit temperature at which martensite is generated by the isothermal transformation, and the point Md is an upper limit temperature at which martensite is generated by the stress inducing transformation. In this case, the above austenite phase is an fcc phase (face centered cubic phase). On the other hand, almost all of the above stress induced-martensite phase is composed of an α′ martensite phase of the bcc phase (body-centered cubic phase), and a very small amount of the ε′ martensite phase of the hcp phase (hexagonal close-packed phase) is contained. The stress induced-martensite phase is defined as the aforementioned α′ martensite phase in this specification, hereinafter.
In the case of a stress inducing martensite transformation, in accordance with increase in an amount of stress induced-martensite, there is a possibility that hardness and brittleness are increased and the mechanical property is changed.
However, as described above, the crystal structure of the austenite phase is different from that of the stress induced-martensite phase. Therefore, the austenite phase stainless steel is a non-magnetic member, and the stress induced-martensite phase stainless steel is a ferromagnetic member, that is, their magnetic properties are greatly different from each other.
Accordingly, when austenite stainless steel is used for a magnetic member or a composite magnetic member described later, it is very effective to increase a ratio of stress induced-ferromagnetic martensite phase.
On the other hand, according to the conventional producing method disclosed in Japanese Unexamined Patent Publication Nos. 7-11397 and 8-3643, it is impossible to increase the magnetic flux density B4000 to a high magnetic level not less than 0.8 T (tesla), wherein the magnetic flux density B4000 is defined as a magnetic flux density in the case of applying a magnetic field with an intensity of 4000 A/m.
The reason why it is impossible to increase the magnetic flux density B4000 to a high magnetic level not less than 0.8 T (tesla) is considered as follows. An amount of strain which can be given to the magnetic member or the composite magnetic member is restricted by the limit at break and the shape of the member. According to the conventional cold working method, even if the maximum strain is given to the magnetic member or the composite magnetic member, a ratio of the generated stress induced-martensite is still low.
For the above reasons, there is a demand for developing a method of positively generating a large amount of stress induced-martensite, that is, there is a demand for developing a method of increasing an amount of the generation of stress induced-martensite with respect to an amount of the strain given to the magnetic member or the composite magnetic member.
Concerning the basic investigation with respect to the method of stress inducing transformation, for example, “Transformation Induced by Working of SUS304 in Various Stress Conditions” was reported in the Spring Lecture Meeting of Plastic Working held in 1995. However, even according to the above investigations, it was impossible to develop the method of generating stress induced-martensite at a high ratio.
In order to solve the above problems, it is a first object of the present invention to provide a method of stress inducing transformation by which stress induced-martensite can be generated in austenite stainless steel at a high ratio of generation, and to provide a method of producing a magnetic member or composite magnetic member, the ferromagnetic property of which is high.
Further, for example, in a device such as an electromagnetic valve having a magnetic circuit, it is necessary to provide parts in which ferromagnetic and non-magnetic portions are integrated with each other. In order to produce such parts having both ferromagnetic and non-magnetic portions, for example, ferromagnetic and non-magnetic parts are separately produced, and then they are integrally connected with each other. However, according to the above production method, the durability of the connecting portion of the ferromagnetic part with the non-magnetic part is not so high, and further the production cost increases.
On the other hand, Japanese Unexamined Patent Publication No. 8-3643 discloses a composite magnetic member and a production method thereof in which ferromagnetic and non-magnetic portions are contiguously formed without having a connecting portion.
As shown in an embodiment described later, the above composite magnetic member can be provided as follows. Austenite alloy steel of a specific composition is used. This austenite alloy steel is subjected to cold working in a predetermined condition so as to generate stress induced-martensite. In this way, the austenite alloy steel is made to be ferromagnetic. After that, desired portions are subjected to solution heat treatment, so that these portions can be made to be non-magnetic.
For example, as shown in FIGS. 22A to 22D, there is provided a composite magnetic member 6 in which the main body is composed of a ferromagnetic portion 2 and the opening side portion is composed of a non-magnetic portion 3. In order to produce the above composite magnetic member 6, first, as shown in FIGS. 15A to 15F explained later, a plate 101 of austenite alloy steel is subjected to pressing by a plurality of times. In this way, the austenite alloy steel plate 101 is formed into a U-shaped member 106 by cold working. Due to the above cold working, stress induced-martensite is generated in the entire U-shaped member 106. Therefore, the entire U-shaped member 106 becomes ferromagnetic. Next, as shown in FIGS. 22A and 22B, the opening side portion of the U-shaped member 106 is subjected to solution annealing by a high frequency induction heating unit 98. Due to the above high frequency induction heating, the opening side portion of the U-shaped member 106 is made to be austenite, that is, a non-magnetic portion 3.
The thus obtained composite magnetic member 6 is excellent in the magnetic property. For example, the magnetic flux density B4000 (the magnetic flux density at H=4000 A/m) of the ferromagnetic portion is not less than 0.3 T, and the specific permeability of the non-magnetic portion μ is lower than 1.2.
However, the following problems may be encountered in the above conventional composite magnetic member 6.
As shown in FIG. 23, stress corrosion cracks 99 tend to occur in the non-magnetic portion 3 close to the boundary between the non-magnetic portion 3 and the ferromagnetic portion 2.
The reason why stress corrosion cracks 99 tend to occur is considered as follows.
As described above, the conventional composite magnetic member 6 is composed of the ferromagnetic portion 2 made of martensite and the non-magnetic portion 3 made of austenite. The crystal structure of austenite and that of martensite are different from each other. Therefore, the density of austenite and that of martensite are different from each other. For the above reasons, the volume of martensite is larger than that of austenite by 3% when the weight of martensite is the same as that of austenite.
In the conventional composite magnetic member 6, material of austenite is used. This material of austenite is transformed into martensite so as to form the ferromagnetic portion 2. Then, a portion of the ferromagnetic portion 2 made of martensite is returned to austenite, so that the non-magnetic portion 3 can be formed. Therefore, as shown in FIGS. 22C and 22D, only the non-magnetic portion 3 is reduced in its volume by 3% compared with the volume of the ferromagnetic portion 2. As a result, residual tensile stress is generated in a portion of the non-magnetic portion 3 close to the boundary between the non-magnetic portion 3 and the ferromagnetic portion 2. It is considered that the generation of this residual tensile stress greatly deteriorates the stress corrosion cracking resistance property.
On the other hand, there is provided another method. As shown in FIGS. 24A to 24C, after the completion of high frequency induction heating for making a portion of the composite magnetic member 6 to be non-magnetic, a punch 96 is forced in the inside of the composite magnetic member 6 so as to expand the non-magnetic portion 3. In this way, the non-magnetic portion 3 is plastically deformed, so that the above residual tensile stress can be removed. However, according to the above method, the following problems may be encountered. As shown in FIGS. 25A to 25C, the size of expanding the non-magnetic portion 3 becomes too large (shown in FIG. 25A) or too small (shown in FIG. 25C), that is, it is difficult to completely control the intensity of residual stress. In order to form the non-magnetic portion 3 into the most appropriate shape as shown in FIG. 25B, it is necessary to control the outer diameter of the punch 96 at a high level of accuracy of 0.01 mm, which is very difficult.
Another conventional method of removing the residual stress is a method of annealing a portion at which the residual tensile stress has been generated. However, in order to completely remove the residual tensile stress generated in the portion close to the boundary between the non-magnetic portion 3 and the ferromagnetic portion 2, it is necessary to anneal the entire composite magnetic member. When the entire composite magnetic member is annealed, the ferromagnetic portion is changed into a non-magnetic portion. Since the performance of the ferromagnetic portion must be maintained in the composite magnetic member, it is impossible to apply the above method.
In view of the above conventional problems, the second object of the present invention is to provide a composite magnetic member and a production method thereof by which the performance of the ferromagnetic portion and the non-magnetic portion can be maintained and it is possible to ensure a high stress corrosion cracking resistance property, as well as to provide an electromagnetic valve made of the above composite magnetic member.